Lupus Anticoagulant Testing in Europe: An Analysis of Results from the First European Concerted Action on Thrombophilia (ECAT) Survey Using Plasmas Spiked with Monoclonal Antibodies against Human β2-Glycoprotein I

Abstract

SummaryLupus anticoagulants (LA) are immunoglobulins directed to either prothrombin or Beta-2-glycoprotein I (β2GPI) bound to phospholipids. Most patients with LA have both β2GPI- and prothrombin-dependent antibodies. Several recent reports have shown that LA is more strongly associated with thrombosis than anticardiolipin antibodies (aCL). Therefore, an accurate detection of LA is of utmost importance in patients suspected of an antiphospholipid syndrome. We recently raised a series of murine monoclonal antibodies against human Beta-2-glycoprotein I (β2GPI) with LA activity similar to affinity purified human β2GPI-dependent LAs. A normal plasma pool, and the same pool spiked with LA positive anti-β2GPI antibodies at two potency levels, were used as materials in an external quality assessment scheme organised by the European Concerted Action on Thrombosis (ECAT). Fifty nine laboratories participating in this trial were asked to test for the presence of a LA in the 3 samples submitted. The majority (82%) of the participants found the high potency LA sample to be positive. Only 37% of the laboratories considered the weak potency LA sample to be positive. The submission of a normal sample, a weakly positive sample and a clearly positive sample enabled us to compare the relative LA responsiveness of the different screening assays used. Clotting time ratios varied from 0.81 to 3.28 for sample B and from 0.66 to 5.32 for sample D. In general, the highest clotting time ratios were found with the dilute prothrombin time (dPT), the dilute Russell Viper Venom time (dRVVT) and the Kaolin Clotting time. The most frequently used screening tests were the aPTT and the dRVVT. With the various assay systems, LA responsiveness varied largely according to the reagents used. For the β2GPI-dependent LA used in this study, PTT LA clearly showed the highest responsiveness among the aPTT reagents and Innovin among the dPT reagents. The present study also shows that many laboratories still rely on poorly responsive screening assays for their LA tests. Other laboratories rely on sensitive and more specific integrated test systems based on a sensitive screening assay with a low phospholipid content and a confirmatory test employing high phospholipid concentrations. The most used integrated system was dRVVT based. However, also here the LA responsiveness was largely reagent dependent.In conclusion, many laboratories still rely on poorly responsive screening assays by which weakly positive LA samples are misdiagnosed. LA positive anti-β2GPI moabs have a potential for the unlimited production of LA control specimens, that may help hemostasis laboratories choose more LA responsive assay systems and to assess intra-laboratory precision of their LA testing.